linux-kernel - Re: [PATCH v5 2/2] drm/tiny: add driver for Apple Touch Bars in x86 Macs

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Message-ID: <28e67cc5-6932-48ac-84ef-93af893b2ed9@suse.de>
Date: Tue, 25 Feb 2025 17:06:37 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Aditya Garg <gargaditya08@...e.com>
Cc: "andriy.shevchenko@...ux.intel.com" <andriy.shevchenko@...ux.intel.com>,
 "maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
 "mripard@...nel.org" <mripard@...nel.org>,
 "airlied@...il.com" <airlied@...il.com>, "simona@...ll.ch"
 <simona@...ll.ch>, Kerem Karabay <kekrby@...il.com>,
 Atharva Tiwari <evepolonium@...il.com>, Aun-Ali Zaidi <admin@...eit.net>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v5 2/2] drm/tiny: add driver for Apple Touch Bars in x86
 Macs

Hi

Am 25.02.25 um 15:54 schrieb Aditya Garg:
[...]
>>>> +static int appletbdrm_probe(struct usb_interface *intf,
>>>> +                const struct usb_device_id *id)
>>>> +{
>>>> +    struct usb_endpoint_descriptor *bulk_in, *bulk_out;
>>>> +    struct device *dev = &intf->dev;
>>>> +    struct appletbdrm_device *adev;
>>>> +    struct drm_device *drm;
>>>> +    int ret;
>>>> +
>>>> +    ret = usb_find_common_endpoints(intf->cur_altsetting, &bulk_in, &bulk_out, NULL, NULL);
>>>> +    if (ret) {
>>>> +        drm_err(drm, "Failed to find bulk endpoints\n");
>>> This is simply wrong (and in this case even lead to crash in some circumstances).
>>> drm_err() may not be used here. That's my point in previous discussions.
>>> Independently on the subsystem the ->probe() for the sake of consistency and
>>> being informative should only rely on struct device *dev,
>> That's never going to work with DRM. There's so much code in a DRM probe function that uses the DRM error functions.
>>
>> This specific instance here is wrong, as the drm pointer hasn't been initialized. But as soon as it is, it's much better to use drm_err() and friends. It will do the right thing and give consistent output across drivers.
>>
> Ok so this is actually an interesting case, since I am trying to fix it. To initialise the drm pointer, we need to initialise adev, and to initialise adev, we need to run usb_find_common_endpoints first. So IMO, we cannot use drm_err here, but rather dev_err_probe can be used.

Maybe start reading those compiler warnings. They are there for a 
reason. Your compiler tells you that you are dereferencing an 
uninitialized pointer, right here:

https://elixir.bootlin.com/linux/v6.13.4/source/include/drm/drm_print.h#L586

Clearing that pointer to NULL will fix the error and make drm_err() work.

Best regards
Thomas

>>
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    adev = devm_drm_dev_alloc(dev, &appletbdrm_drm_driver, struct appletbdrm_device, drm);
>>>> +    if (IS_ERR(adev))
>>>> +        return PTR_ERR(adev);
>>>> +
>>>> +    adev->in_ep = bulk_in->bEndpointAddress;
>>>> +    adev->out_ep = bulk_out->bEndpointAddress;
>>>> +    adev->dmadev = dev;
>>>> +
>>>> +    drm = &adev->drm;
>>>> +
>>>> +    usb_set_intfdata(intf, adev);
>>>> +
>>>> +    ret = appletbdrm_get_information(adev);
>>>> +    if (ret) {
>>>> +        drm_err(drm, "Failed to get display information\n");
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    ret = appletbdrm_signal_readiness(adev);
>>>> +    if (ret) {
>>>> +        drm_err(drm, "Failed to signal readiness\n");
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    ret = appletbdrm_setup_mode_config(adev);
>>>> +    if (ret) {
>>>> +        drm_err(drm, "Failed to setup mode config\n");
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    ret = drm_dev_register(drm, 0);
>>>> +    if (ret) {
>>>> +        drm_err(drm, "Failed to register DRM device\n");
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    ret = appletbdrm_clear_display(adev);
>>>> +    if (ret) {
>>>> +        drm_err(drm, "Failed to clear display\n");
>>>> +        return ret;
>>>> +    }
>>>> +
>>>> +    return 0;
>>>> +}
>> --
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Frankenstrasse 146, 90461 Nuernberg, Germany
>> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
>> HRB 36809 (AG Nuernberg)
>>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)